The subject of the present invention is a process for manufacturing an actuator based on piezoelectric materials, and more particularly an actuator comprising at least one stack of electrode layers and piezoelectric material layers. This actuator may be used in a fluid injector, such as a fuel injector, in the automotive industry.
Fuel injectors are used to produce very fine sprays of fuel in order to improve combustion and reduce pollution emissions. Piezoelectric ultrasonic fluid injectors allow the penetration of fuel into the combustion chamber to be controlled, thus enabling operation with a lean or stratified mixture.
Piezoelectric or magnetostrictive ultrasonic fluid injectors may be used to inject:                diesel or any other fuel in direct injection or prechamber diesel engines;        petrol or any other fuel in what are called direct or indirect injection petrol engines;        any other fuel in an internal combustion engine, or a gas turbine, etc; and        any fluid for regenerating depollution systems, such as for example nitrous oxide traps, or particulate traps, commonly referred to as particulate filters (PFs).        
Ultrasonic injectors produce a fluid spray by breaking up a fluid. Such an injector 1 is illustrated in FIG. 1. The injector 1 comprises an injector body 2 and a fluid entrance 3 through which the fluid enters at a high pressure. The injector 1 makes the fluid pass through an oscillating mechanical device that is an orifice obtained by separating the head 4 of a needle valve from its seat 40. This orifice opens and closes at an ultrasonic frequency. The source of the mechanical excitation is a stack 5 of piezoelectric ceramics or a bar of magnetostrictive material or a material having similar properties. These electroactive materials are supplied with power by suitable power supplies.
The rest of the description is devoted to the active array, i.e. the actuator, comprising the stack of piezoelectric material layers. This active array can be used both in a closed nozzle injector and in an open nozzle injector.
Piezoelectric ceramic stacks conventionally take two forms.
On the one hand, stacks of thick ceramic layers that have a 1 to 5 mm unitary thickness are known in the form of rings or drilled disks allowing a preload screw to pass axially through the stack, which stack also comprises intermediate electrodes made of a metal alloy a few tens or hundred of microns in thickness. This type of assembly gives rise to technical and economic problems in industrial production.
This is because, to be obtain perfect reproducibility, this type of fitting requires additional machining related to the drilling of the parts allowing insertion of the preload screw. This type of fitting also requires each face of each ceramic and each intermediate electrode layer to have submicron flatness, meaning that both faces of each part must be carefully polished before said part is fitted in the stack.
On the other hand, stacks of ceramic layers having a small unitary thickness, typically of about 100 μm (micrometers or microns), and comprising intermediate electrodes a few microns, for example about 5 μm, in thickness, are known, the intermediate electrodes being deposited on the ceramics. In this case, the intermediate electrodes are connected to connection electrodes, for example using a polymer filled with a conductive material, and the area of contact locally makes contact with an intermediate electrode over a height of a few microns, for example 5 μm. This type of assembly causes problems on the one hand because it is difficult to produce the connection, and on the other hand because this connection is mechanically fragile in the contact zone, which does not exceed 5 μm. This assembly is therefore not suitable for generating power ultrasound and does not adequately withstand the presence of liquid or gaseous hydrocarbons because the conductive polymer reacts with the fuel.
Power ultrasound is generated by high-amplitude (approximately about 5 to 10 μm), high-load (approximately about 40 MPa) oscillating movements at ultrasonic frequencies (typically between 10 kHz and 100 kHz). A problem associated with power ultrasound is that it may generate substantial shear stresses at the joint where the intermediate electrode makes contact with the connection electrode.